This invention relates to an improved cooling apparatus, and, more specifically, to an improved cooling assembly for use in fuel cell stacks.
Cross-reference is made to two other copending patent applications pertaining to related subject matter and assigned to the same assignee as this application; application of John Werth entitled "Fuel Cell Crimp-Resistant Coolant Device With Internal Support", Ser. #06/500,464filed on June 2, 1983, now abandoned, and application of Charles Wittel entitled "Fuel Cell Crimp-Resistant Coolant Device With Internal. Coil" serial #06/500,465, filed on June 2, 1983 now U.S. Pat. No. 4,583,583. These applications are incorporated by reference in their entireties herein.
Fuel cell design and operation typically involves conversion of a hydrogen-containing fuel and some oxidant into DC electric power through an exothermic reaction. The chemistry of this reaction is well known and has established parameters and limitations. One such limitation is that the electrochemical reaction produces, as a by-product thereof, substantial waste heat which must be removed in a controlled manner to maintain the cells at their desired operating temperature. For efficient operation, it is generally desirable to maintain the cells at substantially uniform temperature and at a temperature level which is consistent with a controllable rate of reaction of the fuel cells therein.
Conventional methods for removal of waste heat from the fuel cell environment have traditionally involved the use of a laminar heat exchanger assemblies, or cooling assemblies, incorporated within and arranged parallel to the various other layers from which the fuel cells are constructed. Typically, the components of the cooling assembly take the form of passageways which contain a circulating coolant material. The heat generated within the stack is transferred to the coolant as it is circulated through the stack. The coolant is then brought out of the stack and into a heat exchanger where the heat is removed therefrom before the coolant is recirculated through the stack. In this manner the cooling assembly enables control over the temperature of the reaction environment of the fuel cell stack and, thus, its rate and efficiency. The pattern of distribution of the coolant passageways within the stack, their relative size, the heat capacity of the coolant fluid and the volume of coolant which is circulated through the cooling assembly per unit of time determine the heat transfer capacity of the cooling system. Because the cooling system is generally an integral part of the fuel cell stack, it should be electrically isolated from the stack and also should not be adversely affected by corrosive media within the stack such as the hot electrolyte.
The problems associated with corrosion as well as the undesirable flow of electrical current from the stack into the cooling loop are described in detail in U.S. Pat. Nos. 3,964,929; 3,964,930; and 3,969,145. These patents address the problem of the so-called "shunt currents" and attempt to resolve it by electrically insulating the cooling system from ground. This minimizes the driving potential of such currents relative to the coolant. Other techniques for avoiding the problems associated with shunt currents include the use of dielectric coolants.
The heat exchanger configuration described in these patents is rather typical of that employed by the prior art. Generally, the configuration consists of a series of parallel tubes connected to what is generally referred to as a "plenum". The plenum is a reservoir from which coolant is simultaneously distributed into the parallel tubes which are embedded in a fuel cell cooling assembly. After passage of the coolant through the parallel tubes, it is collected in another plenum and, thereafter, returned, through a cooling loop, to the inlet plenum.
The cooling assembly tubes are composed of electrically conductive material such as copper. Water can be used as the coolant and the metal tubes are coated either on their internal or external surfaces with a dielectric material such as polytetrafluoroethylene. This coating is used to reduce the possibility of shunt currents and corrosion of the tubes. The coated tubes are located in passageways formed in the plates of fuel cells in the stack. However, due to manufacturing tolerances, it is difficult to avoid voids such as spaces between the tubes and the walls of the passageways. Since air is a poor conductor of heat, such air spaces can be filled with a thermally-conductive grease which is compatible with the electrolyte to maximize heat transfer from the cells to the coolant. These systems also use a sacrificial anode material at the tube ends to guard against corrosion. In addition, there is the possibility of discontinuities occurring in the Teflon layer such as by manufacturing imperfections, differential thermal expansion, damage during the assembly process, poor bonding, etc. This causes two problems; first the corrosive media in the fuel cell will be able to directly attack the tube and second, the thermal contact will be diminished.
A variation in cooling assembly design is disclosed in U.S. Pat. No. 4,233,369. In this patent, a fibrous, porous coolant tube holder, which also serves as a member through which a reactant gas can travel, is used to hold copper coolant tubes. The tubes, held in channels in the holder, are connected to a coolant inlet header and coolant outlet header. Between the headers, the tubes pass through the stack, make a U-turn and pass back through the stack. The tubes are pressed into the channels and have caulking between the channel walls and tube. In addition to reduced corrosion, this system makes the separator plate thinner and easier to manufacture.
Other techniques are known for bringing coolant materials into a fuel cell stack. For instance, a tubeless system has been used wherein a metal plate is grooved in a pattern on its surface with one or more inlets and outlets. The grooved surface of the plate is then covered with a second ungrooved metal plate, called a brazing sheet, to create an assembly having enclosed coolant passageways and coolant inlet and outlets. In addition, similar passageways can be constructed by assembling two such brazing plates with partitions therebetween which form coolant passageways.
It is evident that the demands upon the cooling systems for fuel cells are significantly greater and more specialized than those encountered by other devices in different heat transfer environments. U.S. Pat. Nos. 1,913,573; 2,819,731; 2,820,615; 2,864,591; and 3,847,194 are illustrative of some of the conventional heat transfer devices found in areas other than the fuel cell-related technologies. In virtually all the heat exchangers described in the immediately foregoing list of patents, the environmental setting contemplated for their use is much more forgiving than that encountered in fuel cells.
Accordingly, it is a principal object of the invention to provide an improved fuel cell cooling assembly.
It is another object of the invention to provide a cooling assembly that maximizes heat transfer from the fuel cell stack to the coolant without undue manufacturing and assembly tolerances.
It is another object of this invention to provide a manifold-free cooling assembly of simplified construction.
It is another object of the invention to provide a cooling assembly which is essentially non-corrosive in the fuel cell environment.
It is another object of this invention to provide a cooling assembly which avoids shunt currents without the need for additional electrical isolation thereof from adjacent fuel cells within the stack.
It is another object of this invention to provide a cooling assembly that can be an integral component of a fuel cell.
It is another object of the invention to provide a cooling assembly that avoids coatings.